The MP for Witney and West Oxon – David Cameron's successor – added: "I had to think who I wanted to see in the mirror for the rest of my life. I cannot tell the people of WOxon that I support the proposals in their current form".

However May's position looks safe for now, with Brexiter MPs telling City A.M. they fear she would survive any vote of no confidence at this stage, which means she could not be challenged again for a year – well over the March exit date – leaving the Prime Minister free rein to get the Brexit terms she wants.

But even if they stay on, Courts will not be the only backbencher to speak out against the current direction of travel.

Former Brexit secretary David Davis is expected to give his resignation speech this afternoon, which is likely to cover the same ground his interviews have done so far – namely that the proposals could leave the UK outside of the EU but still beholden to its rules.

The same piece of legislation, formally known as the Taxation (Cross-Border) Bill, is also expected to face a tough time from Remainers, who have signed an amendment of their own, urging the government to seek a form of customs union after Brexit as part of the negotiations. Although Remainers have so far failed to present a firm front, they have warned that previous battles over the EU Withdrawal Bill were merely a warm up for this week.